Add +2 to SM for anything box-, sphere-, or blob-shaped… For objects much smaller in two of three dimensions (e.g., a rope 100 yards long but just 2” thick), use the smallest dimension instead of the largest when shooting it with a missile or stabbing it with a thrusting attack.

By this rule, I would be at -9 to shoot a 2" think rope, and -7 to shoot a 2" diameter sphere. As missing the rope by a little bit in two directions would actually still hit the rope, the rope should be easier to hit than the sphere, not -2 more difficult.

Proposed houserule: For objects much smaller in two of three dimensions (e.g., a rope 100 yards long but just 2” thick), use the smallest dimension instead of the largest when shooting it with a missile or stabbing it with a thrusting attack, then add +3.

You could try taking the square root of the cross sectional area. For a human that's about half a square yard, so it's square root is about 0.7 yards, so perhaps you need a +1 or +2, which I guess you also need to preserve the modifier for a sphere if you go that route.

Shooting a rope is really hard, and I don't think that it is long actually matters much. For stabbing a rope you can usually stack up to +11 in modifiers in regular GURPS, so it might be worth allowing noncombat melee "attacks" in DFRPG to claim a +10.

Shooting a rope is really hard, and I don't think that it is long actually matters much. For stabbing a rope you can usually stack up to +11 in modifiers in regular GURPS, so it might be worth allowing noncombat melee "attacks" in DFRPG to claim a +10.

Of course it is. That's the point with this rule.

But now I see the point I think Anthony was trying to make. If we shorten the rope, we will eventually come to the point when the rule no longer applies and then it would be easier to hit the rope, even though there is less rope to hit.

It won't be hard enough if you give a huge bonus to compensate, especially if you calibrate it to human proportions and SM 0.

That's a generic issue with the SM system; there should not be a 'rope exception'.

Realistically, a 2" rope is easier to hit than a 2" sphere, because for the rope there is one direction in which scatter matters, for the sphere there are two directions. This should probably be worth about a +2 to hit. Thus, given that hitting a 2" sphere is at -7, the rope should be at -5. If this is a problem, complain about how easy it is to hit the 2" sphere, not how hard it is to hit the rope (obviously, 2" is pretty thick for rope, but that was the OP).